Generally, a digital record signal reproducing apparatus, for example, a digital record signal reproducing circuit of compact disc players has a circuit arrangement, as shown in FIG. 1.
Referring now to FIG. 1, an example of the conventional digital record signal reproducing circuit of compact disc players, will be briefly described. A digital record signal reproduced from a compact disc is applied to a digital-to-analog converter 10 through an input terminal 11. The digital-to-analog converter 10 converts the digital record signal to an analog signal by a conventional manner, as well known to persons skilled in the art. This analog signal is applied to an output amplifier 12 through a low pass filter 13. The low pass filter 13 removes high frequency components from the analog signal. The output amplifier 12 amplifies the analog signal to a prescribed level.
The output amplifier 12 is constructed by, e.g., an operational amplifier 14 and is provided with a feed-back circuit 15. The analog signal passing through the low pass filter 13 is applied to the non-inverse input end (+) of the operational amplifier 14. The output of the operational amplifier 14 is fed back to the inverse input end (-) of the operational amplifier 14 through the feed-back circuit 15.
The feed-back circuit 15 is designed so that the feedback circuit 15 selectively gives the output of the output amplifier 12 a prescribed frequency characteristic. In more detail, the feed back circuit 15 has a first resistor 16 coupled between the output end and the inverse input end (-) of the operational amplifier 14, a second resistor 17 coupled between the inverse input end (-) of the operational amplifier 14 and a ground terminal 18, and a capacitive circuit 19 selectively coupled between the output end and the inverse input end (-) of the operational amplifier 14 in parallel with the first resistor 16. The capacitive circuit 19 is comprised of a capacitor 20, a third resistor 21 and a switch 22 which are connected in series with each other.
In the feed back circuit 15, the first and second resistors 16 and 17 determine the amplification degree of the output amplifier 12. The capacitive circuit 19 becomes effective in the feed back circuit 15 when the switch 22 turns ON. As a result, the output of the output amplifier 12 is given the prescribed frequency characteristics responsive to the capacitance of the capacitor 20, when the switch 22 turns ON. The switch 22 is automatically controlled its ON or OFF by a discrimination circuit 23 for discriminating an emphasis data, as will be described later.
In general, there are two kinds of compact discs in reference to a frequency characteristic of the digital record signal. As is well known to persons skilled in the art, a first kind of compact disc is given a predetermined pre-emphasis frequency characteristic to the digital record signal. The pre-emphasis processing is made to emphasize a higher frequency range of the record signal in a recording process. Compact discs with the predetermined pre-emphasis frequency characteristics are reproduced by predetermined de-emphasis processing to compensate the pre-emphasis frequency characteristics. As a result, the reproduced signal has reduced noise in the higher frequency range.
On the other hand, another kind of compact disc is not given such pre-emphasis processing in the recording process. Compact discs without such a pre-emphasis frequency characteristic are reproduced without any such a de-emphasis processing. That is, compact discs without such a pre-emphasis frequency characteristic are reproduced with processing having a flat frequency response characteristic.
Compact discs have a recording format in which a subcode data, a sort of control signal to be used in a reproducing process of compact discs, carries a prescribed bit for discriminating the pre-emphasis processing and the non pre-emphasis processing in recording process.
In digital record signal reproducing apparatus, as shown in FIG. 1, the apparatus detects the emphasis discrimination subcode data and applies to the emphasis data discrimination circuit 23 a control signal corresponding to the emphasis discrimination subcode data. The switch 22 turns ON or OFF according to the control signal. The switch 22 turns ON when a compact disc with the pre-emphasis processing is reproduced so that the capacitive circuit 19 becomes effective and gives the output of the output amplifier 12 with the de-emphasis frequency characteristic. The switch 22 turns OFF when a compact disc with non pre-emphasis processing is reproduced so that the capacitive circuit 19 is cut off from the feed back circuit 15. As a result, the output of the output amplifier 12 is given the flat frequency response characteristic.
FIG. 2 shows another example of the conventional digital record signal reproducing apparatus. In the circuit of the example, a digital filter 24 is provided prior to the digital-to-analog converter 10. The digital filter 24 reduces high harmonic signals which are close to the basic frequency of the analog output converted by a digital-to-analog converter 10. As a result, the high harmonic load on an analog low pass filter 13 is reduced. Other sections of the circuit are identical with that of FIG. 1.
In the conventional digital record signal reproducing apparatus, the feed back circuit 15 is automatically switched in response to the bit state of the pre-emphasis discrimination subcode data. When the record signal of compact discs with the pre-emphasis processing is reproduced, the feed back circuit 15 is subjected to the de-emphasis processing and the output amplifier 12 gives the output thereof with the de-emphasis frequency characteristics so that the record signal is compensated for the pre-emphasis frequency characteristics. When the record signals of compact discs with the non pre-emphasis processing are reproduced, the feed back circuit 15 is subjected to the flat frequency response characteristic processing and the output amplifier 12 gives the output thereof with the flat frequency response frequency characteristics.
The conventional digital record signal reproducing apparatus however has the following drawbacks. The output amplifier itself generates noises, e.g., thermal noise. The noise level varies when the switch 22 is turned ON or OFF. Further, the conventional digital record signal reproducing apparatus requires an analog switch such as the switch 22 to change the frequency characteristics of the output amplifier 12. The analog switch also generates a noise due to a contact resistance in its contact portions. For reducing the noise due to the contact resistance, a low resistance material is required for contact portions of the analog switch. However, such a material is very expensive.